US President Barack Obama in the Oval Office at the White House in Washington, DC.

When a Philippine government ship evaded a Chinese blockade in disputed waters of the South China Sea last month, a US Navy plane swooped in to witness the dramatic encounter.

The flyover was a vivid illustration of the expanding significance of one of Asia's most strategic regions and underscored a message that senior US officials say President Barack Obama will make in Asia next week: The "pivot" of US military and diplomatic assets toward the Asia-Pacific region is real.

Washington's Asian allies, however, appear unconvinced.

During Obama's four-nation tour of Asia that begins on April 23, his toughest challenge will be to reassure skeptical leaders that the United States intends to be more than just a casual observer and instead is genuinely committed to countering an increasingly assertive China in the region.

Russia's annexation of Ukraine's Crimea peninsula — and perceptions of limited US options to get Moscow to back down — has heightened unease in Japan, the Philippines and elsewhere about whether Beijing might feel emboldened to use force to pursue its territorial claims in the East and South China Seas.

There is also suspicion among some Asian allies that if they come under threat from China, the United States — despite treaty obligations to come to their aid — might craft a response aimed more at controlling damage to its own vital relationship with China, the world's second-biggest economic power.

For Obama, the tricky part of the trip, which will include stops in Japan, South Korea, Malaysia and the Philippines, will be deciding how to set limits on China in a way that soothes US allies in Asia but avoids stoking tensions with Beijing.

"Obama's upcoming visit will be the most critical test of this administration's Asia policy," said Richard Jacobson, a Manila-based analyst with TD International, a business risk and strategic consulting firm.

A sign of anxiety

US officials say the Obama administration's long-promised "rebalancing" of America's economic, diplomatic and security policy toward Asia is on track, largely unaffected by the attention demanded by the crisis in Ukraine or persistent troubles in the Middle East.

The Asia "pivot" — as the White House initially dubbed it — represented a strategy to refocus on the region's dynamic economies as the United States disentangled itself from costly wars in Iraq and Afghanistan.

But doubts about Washington's commitment to Asia are simmering in some allied capitals.

"It was a welcome policy change, but will they do it?" Yukio Okamoto, a former Japanese government adviser on foreign affairs said of the strategic shift toward Asia that Obama announced in 2011. "We do not see any actual sign" of its implementation.

When Obama announced the eastward shift, the most dramatic symbol of the new policy was the planned deployment of 2,500 US Marines in northern Australia, where they would be primed to respond to regional conflicts. It took until this month to build up forces to 1,150 Marines based in Darwin, and the full contingent is not due to be in place until 2017.

"The US pivot towards Asia has had very few tangible, concrete outcomes so far," said Adam Lockyer, a foreign policy and defense analyst at the University of New South Wales.

The administration has promised that the United States will reposition naval forces so that 60 percent of its warships are based in Asia-Pacific by the end of the decade, up from about 50 percent now. But as the US military budget contracts, that likely would represent part of a shrinking US defense pie.

Obama's aides brush aside complaints about the US follow-through on the pivot strategy, saying that no matter how much attention Washington devotes to friends and partners in the region, the allies will always want more from their superpower friend.

"Questions by Asia-Pacific allies about the degree of American commitment has been a constant component of our relationship for 60-plus years. It's not new," said a senior US official, who asked not to be identified because he was not authorized to comment publicly. "It doesn't mean the US won't do more to work with them."